Open Access
Issue
A&A
Volume 612, April 2018
Article Number A58
Number of page(s) 6
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201732266
Published online 24 April 2018

© ESO 2018

Licence Creative Commons
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

The detection and analysis of molecular absorption lines along the lines of sight to background light sources has proven to be an extremely useful tool to investigate the physical and chemical state of the interstellar medium (ISM) thanks to the sensitive formation, destruction, and excitation processes of molecules. Such a technique applies from the solar neighbourhood towards nearby stars (e.g. Savage et al. 1977; Boissé et al. 2013) to the gas in and around high-redshift galaxies revealed by damped Lyman-α systems (DLAs; e.g. Levshakov et al. 1989; Ge et al. 1997; Petitjean et al. 2000; Cui et al. 2005; Srianand et al. 2005; Noterdaeme et al. 2008; Jorgenson et al. 2010; Carswell et al. 2011; Balashev et al. 2017). In addition, the detection of molecular species at high-redshift provides original and sensitive probes of fundamental physics and cosmology. Tiny shifts in the relative wavelengths of H2 Lyman and Werner lines can be used to constrain the possible space-time variation of the proton-to-electron mass ratio down to a few parts-per-million over a timescale of Gyrs (see Ubachs et al. 2016, and references therein). The excitation of CO rotational levels, in turn, provides one of the best thermometers for measuring the temperature of the cosmic microwave background (CMB) radiation at high redshift (Srianand et al. 2008; Noterdaeme et al. 2011). Last but not least, the molecular phase of the ISM makes the link between the gas accreted onto a galaxy and its gravitational collapse that gives birth to stars. However, the small number of known molecular absorbers contrasts with the huge number of DLAs detected so far (e.g. Prochaska et al. 2005; Noterdaeme et al. 2012): only about 25 confirmed high-redshift H2-bearing DLAs have been reported to date (see Balashev et al. 2017 and references therein), highlighting the small covering factor of the molecular gas and the need for efficient selection techniques.

In the local ISM, early works using Copernicus showed that H2 and neutral carbon (C I) were frequently observed in the same absorption systems (e.g. Liszt 1981). Despite the high abundance of carbon, it is usually found in ionised forms in high-redshift DLAs and the neutral carbon is seen only in a small fraction of DLAs that also show H2 absorption (e.g. Ge et al. 2001; Srianand et al. 2005). This is likely due to the first ionisation potential of carbon (11.26 eV) being close to the energy of Lyman-Werner photons that lead to H2 dissociation (through Solomon process, see e.g. Stecher & Williams 1967). C I also conveniently produces absorption lines out of the Lyman-α forest that can be identified even at low spectral resolution. We have therefore performed the first blind survey for neutral carbon lines in quasar spectra from the Sloan Digital Sky Survey (SDSS; Ledoux et al. 2015), without any prior knowledge of the associated atomic and molecular content. The 66 C I candidates constitute our parent sample. We report here on the complete follow up of this sample with the Ultraviolet and Visual Echelle Spectrograph (UVES) at a resolving power R ~ 50 000 and the X-shooter spectrograph (R ~ 5000) at the Very Large Telescope (VLT).

2 Observations and results

We obtained spectra for almost all systems that are observable from Paranal Observatory, that is, a sample of thirty nine confirmed C I absorbers. Details about the observing procedures, data reduction, and metal line measurements are presented in Ledoux et al. (in prep.). A near-infrared study of the Na I and Ca II lines as well as the dust extinction properties are presented in Zou et al. (2018). Here, we focus on the detection of H2 and CO. Wavelengths and oscillator strengths for H2 and CO lines are from the compilations of Malec et al. (2010) and Daprà et al. (2016), respectively.

2.1 Molecular hydrogen

We detect H2 absorption lines whenever covered by our spectra (twelve systems). Five of these are already reported in the literature (Noterdaeme et al. 2007, 2010; Srianand et al. 2008; Jorgenson et al. 2010; Klimenko et al. 2016), from which we have taken the H2 column densities, and seven are new detections. We estimated the total H2 column densities for the new detections through Voigt-profile fitting, focusing on the low rotational levels that contain most of the H2. We note that while the velocity profile of singly ionised metals is wide with a large number of components, we detect H2 only at velocities where C I is also detected. Below we comment on each system, in order of increasing right ascension of the background quasar.

J091721+015448, zabs = 2.107: this system was observed with X-shooter at a spectral resolution of ~60 km s−1. We obtain an accurate measurement of the total H2 column density thanks to the damping wings that are seen for the low rotational levels in the four bands covered by our spectrum (see Fig. 1) and obtain logN(H2) = 20.11 ± 0.06.

J111756+143716, zabs = 2.001: this system is characterised by two narrow H2 components seen in the UVES spectrum (Fig. 2) in different rotational levels. These components also correspond to those seen in the neutral carbon lines. While our best-fit value is found to be around log N(H2) ~ 18, we note that the data quality is poor and that only one band is covered, making it impossible to assess the presence of blends. In addition, at such a column density, the absorption is in the logarithmic part of the curve of growth. We are therefore unable to associate an uncertainty to this measurement that we display with a large arbitrary (albeit quite conservative) 1 dex error bar in Fig. 9.

J131129+222552, zabs = 3.092: thanks to the high absorption redshift, no less than twenty Lyman and Werner H2 bands are covered by our UVES spectrum, shown in Fig. 3. Four components can be distinguished in the high rotational levels but lines from the J = 0 and J = 1 rotational levels are strongly damped and therefore modelled using a single component. The damping wings together with the large number of detected transitions and the achieved high signal-to-noise ratio (S/N) values allow for a very precise measurement of the total H2 column density which we found to be logN(H2) = 19.69 ± 0.01.

J164610+232922, zabs = 1.998: while the S/N of our UVES spectrum in the region of H2 lines (see Fig. 4) is quite low1, two narrow H2 components are clearly seen in rotational levels J = 0–3 and our spectrum covers four Lyman bands that span more than an order of magnitude in oscillator strengths. We find a total column density N(H2) ≈ 1018 cm−2 with a ~30% uncertainty.

J225719−100104, zabs = 1.836: this system is more complex with no less than eight H2 components, strongly blended with each other. Unfortunately, only three Lyman H2 bands are covered by the UVES data (Fig. 5), the bluest of which in a region with low S/N. To remove strong degeneracy between parameters, we had to fix the excitation temperature T01 to 100 K. While this is a strong assumption, we note that varying T01 within a factor of two has little effect on the total column H2 density (changes ~0.1 dex). Still, we caution that this error may be underestimated and covering bluer transitions is required to confirm our column density measurement ( log N(H2) = 19.5 ± 0.1).

J233156-090802, zabs = 2.142: in spite of the low S/N achieved for this system, shown in Fig. 6, the data is clearly consistent with strongly damped H2 lines at the same redshift as that of CO lines (see following section). We fitted the J = 0, 1, 2 lines, from which we obtain realistic excitation temperatures, T01 ~ 140 K and T02 ~ 180 K. The total H2 column density is found to be logN(H2) = 20.57 ± 0.05.

J233633-105841, zabs = 1.829: the H2 profile in this system is well modelled by two components, that are partially blended at the X-shooter spectral resolution. The bluest component, however, dominates the total column density, and the measurement is facilitated by the presence of damping wings and the high S/N achieved. We note that the L0–0 band is partially blended with unrelated absorption lines, which we modelled when fitting H2 (see Fig. 7). We obtain logN(H2) = 19.0 ± 0.12.

thumbnail Fig. 1

Selected regions of the X-shooter spectrum of J0917+0154 featuring H2 lines. The rotational levels J = 0 to J = 2 are indicated in blue above their corresponding absorption line and the label above each panel indicates the band they belong to. Higher rotational levels are fitted but not labelled to avoid overcrowding the figure.

Open with DEXTER
thumbnail Fig. 2

As in Fig. 1 but for the UVES spectrum of J1117+1437.

Open with DEXTER
thumbnail Fig. 3

As in Fig. 1 but for the UVES spectrum of J1311+2225. We note that different bands start to overlap which each other at the shortest wavelength. As for other systems, the label indicated at the top left of each panel corresponds to the band to which the identified rotational levels belong.

Open with DEXTER

2.2 Carbon monoxide

CO is detected in seven systems in our sample, six of them already reported by our group and one being a new detection presented here for the first time. This brings the number of known high-z CO-bearing quasar absorbers to nine2. We measured upper limits on N(CO) for all systems assuming the Doppler parameter to be 1 km s−1, similar to what has been measured in all high-z CO absorbers to date. We also assume the CMB radiation to be the main excitation source in diffuse gas at high-z (as observed by Srianand et al. 2008; Noterdaeme et al. 2011).

We calculated the local (i.e. for each band individually) and global χ2 values for a range of total column densities. CO is detected when the χ2 curves are consistent with each other and present a clear inflexion point, defining the best-fit value. For non-detections, χ2 (N(CO)) is generally monotonic with a minimum consistent with that of N(CO) = 0 within uncertainty. Our 3 σ upper limit corresponds to the column density where the χ2 is 9 above this minimum. With this method, not only do we recover all the known CO absorbers but we also identify the new CO system, at z = 2.143 towards SDSS J2331−0908 (Fig. 8), observed by Nestor and collaborators (Prog. ID 080.A-0795). This is only the fourth high-z system with direct and simultaneous measurements of N(CO) and N(H2).

Before discussing our findings, it is worth mentioning that, in the local ISM, the excitation temperature of CO is found to be a few degrees above the CMB temperature (e.g. Burgh et al. 2007), owing to additional excitation processes such as collisions, far-infrared dust emission, and possibly cosmic rays. Relaxing our assumptions we find that the derived CO column density limits (as well as the CO column density for the new detection at zabs = 2.143 towards SDSS J2331−0908) are not changed significantly as the total band equivalent width is almost conserved. For example, allowing an excitation temperature5 K above the CMB temperature only increases the derived values by less than 0.04 dex.

thumbnail Fig. 4

As in Fig. 1 but for the UVES spectrum of J1646+2329.

Open with DEXTER
thumbnail Fig. 5

As in Fig. 1 but for the UVES spectrum of J2257−1001.

Open with DEXTER
thumbnail Fig. 6

As in Fig. 1 but for the UVES spectrum of J2331−0908. The data have been rebinned by 7 pixels for visual purposes only.

Open with DEXTER
thumbnail Fig. 7

As in Fig. 1 but for the X-shooter spectrum of J2336-1058. The green and purple dotted lines in the bottom panel show the contribution from unrelated Lyα (from a sub-DLA at zabs = 1.585) and O VI (zabs = 2.039) absorption, respectively. The contribution from H2 alone is shown in red and the total absorption-line profile is depicted in orange.

Open with DEXTER

3 Discussion

Table 1 summarises the H2 and CO detections and column density measurements. Figure 9 presents the H2 and CO column densities as well as the overall molecular fraction as a function of N(C I) for our complete sample. Known systems from the literature are also added for comparison but not considered for statistical analysis.

We find that H2 is detected with N(H2)≳1018 cm−2 in all systems with logN(C I) > 13.5. In this regime, H2 is likely to be self-shielded and the molecular fraction substantial in the H2 -bearing gas. We also observe a possible trend for increasing N(H2) with increasing N(C I) (Spearman rank correlation coefficient r = 0.4, 1.2 σ significance)in our statistical sample, albeit with a large dispersion. We note that systems that were not C I-selected (from literature) seem not to follow this trend. Four of them indeed have N(H2) > 5 × 1019 cm−2 in spite of relatively low C I column density ( logN(C I)≲14). This difference is likely due to the different chemical enrichments: C I-selected systems probe mostly high-metallicity gas (Zou et al. 2018; Ledoux et al., in prep.) while the four mentioned literature systems all have low metallicities.

Since the column density at which H I is converted into H2 strongly depends on the chemical properties of the gas, in particular the abundance of dust grains (e.g. Bialy & Sternberg 2016), we can expect less H I in the molecular cloud envelope for high-metallicity systems compared to low-metallicity ones. In addition, contrary to DLAs, C I systems were selected without any prior knowledge of the H I content (Ledoux et al. 2015)and should have less contribution from unrelated atomic gas that does not belong to the envelope of the H2 cloud. This is seen in the bottom panel of Fig. 9, where the correlation between and N(C I) is seen with r = 0.6 at 2.1 σ: the average overall H2 molecular fraction is about 15% in our sample (and about 30% when CO is detected) but <3% at log N(C I) < 14.

The correlation between N(CO) and N(C I) for CO detections is strong with r = 0.88 (2.6 σ). From the column density distributions, we can see that the probability to detect CO becomes much larger above N(C I) ~ 5 × 1014 cm−2 (6/12) than below this value (1/27). In addition, there is no CO detection among the 18 systems with log N(C I) < 14.4. Since the CO detection limits are significantly below ( ~1 dex) the typical N(CO) in the case of detection, this result is robust3. Considering also lower and upper limits on both N(CO) and N(C I), we still find the N(CO)–N(C I) correlation to have ~92% probability.

This strong correlation is likely due to the strong dependence of CO abundance on the metallicity (through the abundance of carbon, the abundance of dust grains as molecule-formation catalyst, and an effective shielding of UV photons). In Ledoux et al. (2015), we showed that strong C I systems produce more reddening than other classes of quasar absorbers. We further note that the dust reddening is systematically higher in CO-bearing systems than other H2 systems without CO. The relative behaviours of CO, H2, and C I agree qualitatively with models of ISM clouds immersed in a UV radiation field: these clouds are expected to exhibit an onion-like structure where hydrogen converts from atomic to molecular form when going towards the centre of the cloud. Carbon is predominantly ionised in the external layers, then becomes neutral, while CO is dominant only in the inner dense molecular parts of the cloud (see e.g. Bolatto et al. 2013). Unfortunately, it remains difficult to disentangle the atomic gas that belongs to a molecular cloud envelope and contributes to its shieldingfrom unrelated H I, simply intercepted along the same line of sight. This means that the measured is a lower limit to the actual H2 molecular fraction in the C I-bearing cloud. Since CO and C I are only found in shielded gas, their observed abundance ratio should be less affected by the presence of unrelated gas. Indeed, we find CO/C I ~ 0.1 for all detections (green dotted line in Fig. 9), a value which is also consistent with the non-detections at lower N(C I). This indicates a regime deeper than the layer where the H I–H2 transition occurs.

The CO/H2 ratio is found to be low (~[39] × 10−6) for three out of four cases where both these molecules are detected and can be more than an order of magnitude lower in other strong H2 systems, including the new CO detection. Even in these cases, the high N(H2) likely indicates well-molecularized regions. Several factors such as the grain size distribution or the intensity of the cosmic-ray field likely play important roles in determining whether CO will be present or not in H2 -dominated regions (e.g. Shaw et al. 2016; Noterdaeme et al. 2017; Bisbas et al. 2017). Multiple clouds can also easily explain large H2 column densities without significant CO, in a similar way to how multiple H I–H2 transition layers explain higher N(H I) than predicted by single cloud models (Bialy et al. 2017).

We conclude that C I is a very good proxy to spot high-redshift molecular absorbers that can be used for a variety of studies including fundamental physics and cosmology. It is however crucial not only to constrain the physical parameters in individual systems (and hopefully for individual velocity components separately) but also to explore different metallicity regimes (Balashev et al. 2017) using different selections (e.g. Balashev et al. 2014) to understand better the molecular structure of ISM clouds at high redshifts.

thumbnail Fig. 8

CO AX bands at zabs = 2.1422 towards J2331−0908 (top four panels, AX(3–0) is not covered by the instrument setup). The bottom-right panel shows a co-addition of the CO bands, using -weighting, where f is the oscillator strength and σ the uncertainty on the normalised flux, for easy visualisation of the detection. The bottom-left panel shows the curves (greyfor individual band, black for total).

Open with DEXTER
Table 1

CO and H2 content of strong C I absorbers.

thumbnail Fig. 9

Column densities of CO (top), H2 (middle) and overall molecular fractions (bottom) vs N(C I). CO detections are represented by red colours. The N(C I)-distributions and median logN(CO) values (horizontal lines) are shown for the statistical sample only (circles). Squares correspond to high-z H2 DLA systems from the literature (Balashev et al. 2010, 2011, 2017; Carswell et al. 2011; Guimarães et al. 2012; Noterdaeme et al. 2015, 2017; Petitjean et al. 2002).

Open with DEXTER

Acknowledgements

We thank T. Krühler for help with the X-shooter data reduction. P.N. thanks the European Southern Observatory for hospitality and support during part of this work was done. P.N., P.P.J. and R.S. acknowledge support from the Indo-French Centre for the Promotion of Advanced Research (Project 5504-B). We acknowledge support from the PNCG funded by CNRS/INSU-IN2P3-INP, CEA and CNES, France. This research is part of the project HIH2 funded by the Agence Nationale de la Recherche, under grant ANR-17-CE31-0011-01 (JCJC). S.B. thanks the Institut d’Astrophysique de Paris for hospitality and the Institut Lagrange de Paris for financial support. S.L. has been supported by FONDECYT grant 1140838 and by PFB-06 CATA.

References


1

Although SDSS J164610+232922 is a relatively bright quasar (g = 18.5), only a single 4000 s exposure could be obtained at a high airmass (1.7).

2

The detections towards J1211+0833 (Ma et al. 2015) and J0000+0015 (Noterdaeme et al. 2017) are not formally part of the statistical sample although selected upon their C I content.

3

We note that N(CO)-limits also tend to be more stringent for systems with low N(C I). This is likely due to easier detection of small C I equivalent widths in SDSS towards bright quasars (or that such systems extinguish less the background quasars), for which the follow-up data is also of better quality.

All Tables

Table 1

CO and H2 content of strong C I absorbers.

All Figures

thumbnail Fig. 1

Selected regions of the X-shooter spectrum of J0917+0154 featuring H2 lines. The rotational levels J = 0 to J = 2 are indicated in blue above their corresponding absorption line and the label above each panel indicates the band they belong to. Higher rotational levels are fitted but not labelled to avoid overcrowding the figure.

Open with DEXTER
In the text
thumbnail Fig. 2

As in Fig. 1 but for the UVES spectrum of J1117+1437.

Open with DEXTER
In the text
thumbnail Fig. 3

As in Fig. 1 but for the UVES spectrum of J1311+2225. We note that different bands start to overlap which each other at the shortest wavelength. As for other systems, the label indicated at the top left of each panel corresponds to the band to which the identified rotational levels belong.

Open with DEXTER
In the text
thumbnail Fig. 4

As in Fig. 1 but for the UVES spectrum of J1646+2329.

Open with DEXTER
In the text
thumbnail Fig. 5

As in Fig. 1 but for the UVES spectrum of J2257−1001.

Open with DEXTER
In the text
thumbnail Fig. 6

As in Fig. 1 but for the UVES spectrum of J2331−0908. The data have been rebinned by 7 pixels for visual purposes only.

Open with DEXTER
In the text
thumbnail Fig. 7

As in Fig. 1 but for the X-shooter spectrum of J2336-1058. The green and purple dotted lines in the bottom panel show the contribution from unrelated Lyα (from a sub-DLA at zabs = 1.585) and O VI (zabs = 2.039) absorption, respectively. The contribution from H2 alone is shown in red and the total absorption-line profile is depicted in orange.

Open with DEXTER
In the text
thumbnail Fig. 8

CO AX bands at zabs = 2.1422 towards J2331−0908 (top four panels, AX(3–0) is not covered by the instrument setup). The bottom-right panel shows a co-addition of the CO bands, using -weighting, where f is the oscillator strength and σ the uncertainty on the normalised flux, for easy visualisation of the detection. The bottom-left panel shows the curves (greyfor individual band, black for total).

Open with DEXTER
In the text
thumbnail Fig. 9

Column densities of CO (top), H2 (middle) and overall molecular fractions (bottom) vs N(C I). CO detections are represented by red colours. The N(C I)-distributions and median logN(CO) values (horizontal lines) are shown for the statistical sample only (circles). Squares correspond to high-z H2 DLA systems from the literature (Balashev et al. 2010, 2011, 2017; Carswell et al. 2011; Guimarães et al. 2012; Noterdaeme et al. 2015, 2017; Petitjean et al. 2002).

Open with DEXTER
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.